1. Field of the Invention
The present invention relates to a switching power supply unit for connecting an output obtained by switching a direct current input voltage to an output winding of a power converting transformer and to a voltage converting method.
2. Description of the Related Art
Hitherto, various types of switching power supply units have been proposed and provided for practical use. Many of them are of a type in which a direct current input voltage is switched by switching operation of a switch circuit connected to an input winding of a power converting transformer, and the switched output is connected to an output winding of the power converting transformer. A voltage appearing in the output winding in association with such switching operation of the switch circuit is rectified by a rectifier circuit, the rectified voltage is converted to a direct current voltage by a smoothing circuit, and the smoothed voltage is output.
In a switching power supply unit of this kind, an output rectifier device such as an output rectifier diode is connected in series with a power transmission line in the rectifier circuit. Therefore, to improve efficiency of the switching power supply unit, it is extremely effective to reduce a loss in the output rectifier diode.
To reduce a loss in the output rectifier diode, it is sufficient to use a diode having a small forward voltage drop. However, the diode having a small forward voltage drop has also a low reverse withstand voltage. Consequently, in the case of using a diode having a small forward voltage drop as the output rectifier diode, it is particularly necessary to suppress the reverse voltage.
In a switching power supply unit of this kind, a reverse voltage which has to be considered the most is a surge (spike) voltage caused by a parasitic element accompanying on/off operation of the switch circuit. The surge voltage is applied as a reverse voltage to the output rectifier diode. As means for suppressing the surge voltage, conventionally, a so-called snubber circuit is known.
For example, the applicant of the present invention has proposed a snubber circuit utilizing LC resonance in Japanese Patent Publication No. 3,400,443. The snubber circuit can suppress the surge voltage to a predetermined voltage or less by utilizing LC resonance.